🏷️ ensures that handler return is properly typed

- This change addresses an issue where the return type of handlers was not being correctly enforced by the TypeScript compiler. As far as I could tell, the issue is caused by the `infer` keyword not working with generic types.
- Added `TResult` as a type parameter to `MiddlewareHandler` to explicitly specify the handler's return type.
- Modified the `middy` function signature to pass `TResult` to `MiddlewareHandler`.
- By explicitly specifying `TResult`, we ensure that the compiler can correctly enforce the handler's return type.

closes #1228

packages/core/index.d.ts

@@ -99,7 +99,7 @@ export interface MiddyfiedHandler<
   handler: <TInputHandlerEventProps = TEvent, TInputHandlerResultProps = TResult>(
     handler: MiddlewareHandler<
     LambdaHandler<TInputHandlerEventProps, TInputHandlerResultProps>,
-    TContext
+    TContext, TResult
     >
   ) => MiddyfiedHandler<TInputHandlerEventProps, TInputHandlerResultProps, TErr, TContext, TInternal>
 }
@@ -150,8 +150,9 @@ infer TMiddlewareInternal
 
 declare type MiddlewareHandler<
   THandler extends LambdaHandler<any, any>,
-  TContext extends LambdaContext = LambdaContext
-> = THandler extends LambdaHandler<infer TEvent, infer TResult> // always true
+  TContext extends LambdaContext = LambdaContext,
+  TResult = any
+> = THandler extends LambdaHandler<infer TEvent, TResult> // always true
   ? MiddyInputHandler<TEvent, TResult, TContext>
   : never
 
@@ -169,7 +170,7 @@ declare function middy<
 > (
   handler?:
   | LambdaHandler<TEvent, TResult>
-  | MiddlewareHandler<LambdaHandler<TEvent, TResult>, TContext>
+  | MiddlewareHandler<LambdaHandler<TEvent, TResult>, TContext, TResult>
   | PluginObject,
   plugin?: PluginObject
 ): MiddyfiedHandler<TEvent, TResult, TErr, TContext, TInternal>

packages/core/index.test-d.ts

@@ -408,4 +408,19 @@ const s3Handler = async (event: S3Event): Promise<void> => {
 }
 
 const handler1182 = middy().handler(s3Handler)
-expectType<MiddyfiedHandler<S3Event, void, Error, Context, {}>>(handler1182)
+expectType<MiddyfiedHandler<S3Event, any, Error, Context, {}>>(handler1182)
+
+//  Issue #1228 Correct return type
+const numberHandler = middy<APIGatewayProxyEvent, number>()
+  .handler(async (event) => {
+    return 42; // Correct return type, should pass type checking
+  });
+expectType<middy.MiddyfiedHandler<APIGatewayProxyEvent, number>>(numberHandler);
+
+//  Issue #1228 Incorrect return type
+const invalidNumberHandler = middy<APIGatewayProxyEvent, number>()
+  // @ts-expect-error
+  .handler(async () => {
+    return 'not a number';
+  });
+expectType<middy.MiddyfiedHandler<APIGatewayProxyEvent, number>>(invalidNumberHandler);